Preparation of Nb-doped TiO2 nanopowder by liquid-feed spray pyrolysis followed by ammonia annealing for tunable visible-light absorption and inhibition of photocatalytic activity

Abstract Visible-light absorbing TiO2 with color tunability and negligible photocatalytic activity was developed by synthesizing Nb-doped TiO2 using liquid-feed flame spray pyrolysis (LF-FSP) followed by NH3 annealing. Nb-doped TiO2 nanopowders with selected Nb contents were synthesized by changing ratios of metalloorganic precursors. By using Ti and Nb precursors whose thermal decomposition behavior is very similar, the maximum Nb doping concentration was improved compared to Nb-doped TiO2 prepared by flame spray pyrolysis (FSP) in previous reports. As-synthesized Nb-doped TiO2 nanopowders were white regardless of the Nb content, while all became colored after NH3 annealing. In addition, the color of Nb-doped TiO2 after NH3 annealing varied from yellow to dark green depending on the Nb content. Light absorption characterization indicates that the color variation is attributable to the localized levels formed by interstitial N-doping and Nb-doping-induced Ti3+ formation resulting from reduction by H2, formed as a result of NH3 decomposition.